One of the most important properties for a separator used in electrochemical devices such as a variety of batteries and an electrical double layer capacitor, includes electrolyte holding properties. An electrochemical device comprising a separator having poor electrolyte holding properties has an increased internal resistance, so that the device encounters problems of a lack of the capacity, lowering of the voltage, and shortening of the life. As a separator for lithium primary or secondary battery, for example, Japanese Prov. Patent Publication No. 105851/1991 discloses “a separator for lithium battery, comprising a microporous film formed from a polyethylene composition which comprises 1% by weight or more of ultra-high molecular weight polyethylene having a weight average molecular weight of 7×105 or more, and which has a ratio of weight average molecular weight to number average molecular weight of 10 to 300, wherein the microporous film has a thickness of 0.1 to 25 μm, a void rate of 40 to 95%, an average through-hole diameter of 0.001 to 0.1 μm, and a breaking strength of 0.5 kg or more as measured with respect to a 10-mm width specimen”.
However, a separator of this type has a pore diameter as extremely small as submicron or less, and therefore, when an electrolyte has a high viscosity, the electrolyte is difficult to penetrate the separator, resulting in a poor battery-assembly efficiency. In addition, in this separator, the pores are linearly formed in the Z direction. Therefore, the electrolyte holding capacity of the separator is more or less lowered, and the electrodes gradually swell while repeating charge-discharge operations and the separator is pressed by the swelled electrodes to cause the electrolyte to ooze from the separator, gradually lowering the capacity of the device.
For solving the problem, it has been recently proposed to use nonwoven fabric having excellent gas permeability as a separator. In nonwoven fabric, individual fibers are relatively randomly stacked on and adhere to one another in the Z direction of the fabric. Therefore, the pores formed in the nonwoven fabric are not linear and thus the nonwoven fabric advantageously has excellent electrolyte holding properties. However, pinholes are likely to be formed in thinner nonwoven fabric, and dry nonwoven fabric especially has disadvantages in that the electrolyte holding properties are rather unsatisfactory and uniform thickness is difficult to obtain. As a method for solving the problems, a way of employing wet paper making using fibrillated or microfibrillated fibers is effective.
For example, in Japanese Prov. Patent Publication No. 27311/1997, the present inventors have disclosed “nonwoven fabric for a battery separator, comprising organic fibers which are at least partially fibrillated into a fiber diameter of 1 μm or less and having a gas permeability of 100 mmHg or more”, in view of providing nonwoven fabric for a battery separator, which has excellent gas permeability and excellent electrolyte holding properties and which suffers no formation of pinholes and can prevent internal short-circuit.
In electrochemical devices, such as a primary battery, a secondary battery, an electrolytic capacitor, and an electrical double layer capacitor using an organic electrolyte or a nonaqueous gel electrolyte as an electrolyte or a gel electrolyte, a slight amount of moisture contained in the system of the device leads to problems of lowering of the capacity and shortening of the life of the device. Therefore, greatest possible care is taken so that no moisture is contained in the device.
Specifically, in the production process of electrochemical devices, electrodes and a separator are generally dried at a high temperature or dried in vacuum at a high temperature after laminating and winding together, to remove the moisture contained in the electrodes and separator. For example, with respect to the electrical double layer capacitor, drying temperature is generally 150° C. or higher, and recent trend is to elevate the drying temperature to as high as 180° C. or 200° C., for improving drying efficiency, and hence a separator having excellent heat resistance is desired.
For example, Japanese Prov. Patent Publication No. 68380/2001 discloses “a separator for electrical double layer capacitor using an aqueous electrolyte and comprising a separator disposed between a pair of polarizable electrodes, wherein the separator comprises nonwoven or woven fabric comprised mainly of sulfonated polyolefin fibers”. When this separator is exposed to a temperature of 150° C. or higher for a long period of time, the polyolefin fibers in the separator melt, so that the separator cannot maintain its form. Thus, the separator does not have sufficient heat resistance, which is one of the objectives that we intend to attain.
Japanese Prov. Patent Publication No. 45586/1997 discloses “an electrical double layer capacitor comprising a pair of polarizable electrodes, a separator disposed between the polarizable electrodes, and an electrolyte, wherein the polarizable electrodes and the separator are impregnated with the electrolyte, wherein solvent-spun rayon and sisal pulp which is natural fiber are used as raw materials for the separator and subjected to paper making to produce the separator”, and it is clearly described that, when the separator does not contain 30% or more of the solvent-spun cellulose fibers and 40% or more of the sisal pulp, excellent electrical properties of the capacitor cannot be obtained. Japanese Prov. Patent Publication No. 3834/2000 discloses “an electrolytic capacitor comprising a separator disposed between a pair of polarizable electrodes, wherein the separator is produced by paper making using, as a raw material for the separator, 60 to 100% by weight of a beat material comprising beatable solvent-spun cellulose”.
When a separator of paper type comprising the above-mentioned blend of rayon (cellulose fibers) and pulp or solvent-spun cellulose fibers is dried at a temperature as high as 180° C. or 200° C. for a long period of time, the separator suffers deterioration by carbonization. Therefore, the separator must be dried at a low temperature of about 150° C. for as long as about one day, thereby lowering the production efficiency of electrochemical devices. Further, when the separator which has been dried at a high temperature is allowed to stand in an ambient atmosphere, the separator absorbs moisture and returns in a short time to a state before drying. Therefore, the separator has another problem that an electrochemical device comprising the separator is likely to suffer lowering of the capacity and shortening of the life. Thus, the above separators do not have sufficient heat resistance, which is one of the objectives that we intend to attain.
In the production process for electrochemical devices, when electrodes and a separator are wound together, a high tension may be applied to the separator depending on the type of the winding machine used, causing the separator to be broken. Further, depending on the electrodes, the fin of the electrode base material may penetrate and puncture the separator. For this reason, the separator is required to have sufficient tensile strength and puncture strength.
For example, Japanese Patent No. 2965335 discloses “an electrolytic capacitor comprising a separator disposed between an anode foil and a cathode foil, wherein the separator is a separator for electrolytic capacitor, comprising mixed nonwoven fabric comprising glass fiber, and heat resistant organic polymer fibers having a glass transition temperature of 130° C. or higher and having a welding property at a melting point or glass transition temperature thereof or higher”. Glass fiber has no binding ability, and therefore, in this separator for electrolytic capacitor, the heat resistant organic polymer fibers are used as a binder such that they are fused by heat to other fibers. On the other hand, the wet nonwoven fabric in the present invention comprises liquid crystalline polymer fibers having a melting point or a heat decomposition temperature of 250° C. or higher, but the separator is not subjected to thermal treatment at a temperature at which the fibers melt, which means the fibers are not positively fused melt and fused to other fibers.
Like the separator for electrolytic capacitor disclosed in the above patent document, when the separator solely comprises glass fiber and heat resistant organic polymer fibers which are used as a binder to be fused by heat, it is difficult to fuse the polymer fibers by heat only at contact points between the glass fibers and the heat resistant organic polymer fibers or contact points between the heat resistant organic polymer fibers themselves. Instead, the whole of the heat resistant organic polymer fibers are likely to be melted to form a film. Even if the heat resistant organic polymer fibers can be fused only at contact points between the fibers, in the region having a high density of the heat resistant organic polymer fibers, the organic polymer fibers are likely to be fused by heat to form a film, leading to problems that the formation and thickness distribution of the resultant mixed nonwoven fabric are not uniform, and that the electrolyte holding properties of the separator become poor. All the mixed nonwoven fabric described in the working examples of the above patent document has a glass fiber content of 40% or more. Therefore, when a separator comprising such nonwoven fabric is bent, the separator is likely to be broken at the bending portion, and hence it poses a problem of winding properties.
Japanese Prov. Patent Publication No. 40131/1999 discloses “a battery separator comprising a sheet prepared by mixing and shaping a stock comprising at least thermoplastic polymer pulp having a melting point of 200° C. or lower and an organic compound having substantially no stable melting temperature”. As can seen in the working examples, when this battery separator is exposed to a temperature of 120° C. or higher for a period of time as short as three minutes, the thermoplastic polymer pulp is melted to form a film, so that the pores in the separator are blocked to lower the gas permeability. When the separator having such properties is dried, together with electrodes, at a temperature as high as 150° C. or higher, 180° C. or higher, or 200° C. or higher, for a long period of time, the gas permeability of the separator is lowered, markedly increasing the internal resistance of the electrochemical device is, thereby considerably lowering the function of the electrochemical device. Therefore, the separator does not have sufficient heat resistance, which is one of the objectives that we intend to attain.
Japanese Prov. Patent Publication No. 321785/1998 discloses “a separator for alkaline battery, comprising 5 to 80% by weight of paraaramid fibers having a freeness of 400 ml or less, as all of or part of the synthetic fiber component, and 20 to 60% by weight of a cellulose fiber component, based on the total weight of the fibers, which has a center line average surface roughness of less than 10 μm, a maximum pore diameter of 35 μm or less, and an average pore diameter of 5 to 20 μm”. Such a separator comprising 20% or more of a cellulose component has high moisture absorption, and therefore has a low drying efficiency, and, when this separator is dried at a higher temperature, it suffers deterioration by carbonization. Therefore, the separator does not have sufficient heat resistance, which is one of the objectives that we intend to attain. Further, the separator having a maximum pore diameter of 35 μm and an average pore diameter of 5 to 20 μm is not appropriate at all as a separator for electrochemical device having an electrode comprising an active material having an average particle diameter of about several μm. Thus, the separator does not have sufficient internal short-circuit preventing properties, which are one of the objectives that we intend to attain.
Japanese Prov. Patent Publication No. 35754/2001 discloses “a separator for electrical double layer capacitor using sulfuric acid as an electrolyte and comprising a pair of polarizable electrodes, and a separator disposed between the electrodes, wherein the separator comprises wet nonwoven fabric comprised mainly of acid resistant single fibers and acid resistant fibrillated fibers, and wherein the separator has a thickness of 300 μm or less, a void rate of 40 to 90%, and a liquid holding rate of 200% by weight or more, based on the weight of the separator, as measured by 40% sulfuric acid impregnation”. That is, disclosed is a separator for electrical double layer capacitor, which is exclusively used in an electrical double layer capacitor wherein the electrolyte is sulfuric acid and the electrodes are a pair of polarizable electrodes. This separator is improved mainly in the sulfuric acid holding rate by using acid resistant fibers, and an improvement of the heat resistance which is one of the objectives of the present invention is not taken into consideration at all. In fact, in the working examples of the above patent document, a separator for electrical double layer capacitor comprising 100% of acrylic fibers and a separator comprising 100% of polyolefin fibers are exemplified, irrespective of fiber forms, and these separators do not have sufficient heat resistance, which is one of the objectives of the present invention.
The present invention has been made for solving the above-mentioned problems accompanying conventional techniques. Specifically, an object of the present invention is to provide a separator for electrochemical device, which has excellent heat resistance, excellent electrolyte holding properties and excellent internal short-circuit preventing properties as well as excellent winding properties, and which can lower the internal resistance of an electrochemical device and prolong the life of the device, and a method for producing the separator and an electrochemical device.